1. Technical Field
The present invention relates to a Coriolis flowmeter and, more particularly, to a Coriolis flowmeter that determines a state of air bubbles in a fluid subject to measurement (hereinafter called a “target fluid”).
2. Description of the Related Art
A Coriolis flowmeter is used in measuring a flow rate of a target fluid in flow rate control performed in a chemical plant, or the like. A configuration of an example Coriolis flowmeter is illustrated in JP-A-8-29229.
In addition to being able to measure a flow rate, the Coriolis flowmeter can also detect air bubbles in a target fluid, and a method for detecting air bubbles is described in JP-T-2002-525623. The method for detecting air bubbles is described by reference to FIG. 9. FIG. 9 is a view showing a time variation in drive gain of a drive circuit that activates a measurement tube of a Coriolis flowmeter sensor.
When air bubbles have got into a target fluid, a loss in drive energy of the measurement tube increases, so that a vibration amplitude of the measurement tube becomes smaller. In this case, in order to hold a predetermined vibration, the drive gain of the drive circuit and a drive voltage output from the drive circuit increase. When the drive gain exceeds a threshold value, mixing of air bubbles is detected.
In FIG. 9, a threshold value for detecting air bubbles is taken as 606. Air bubbles are not mixed in a target fluid during a period from time t0 to time t1, and the drive gain is smaller than the threshold value 606.
Air bubbles begin to get into the target fluid from time t1 and are mixed in the target fluid during a period of time 600. Consequently, the drive gain exceeds the threshold value 606 during this period, and mixing of the air bubbles is detected.
However, when a pressure of the target fluid is high, diameters of air bubbles become smaller than those achieved when the pressure is low. Therefore, a loss in drive energy becomes smaller, and the drive gain decreases during a period of time 600 shown in FIG. 9.
When an average drive gain achieved during the period of time 600 becomes slightly larger than the threshold value 606, the drive gain goes up and down relative to the threshold value 606 because the drive gain is fluctuating. Fluctuation of the drive gain leads to a problem of repeated presence and absence of air bubbles occurring during the period of time 600, which in turn makes it impossible to accurately detect air bubbles.